Needleless injectors are used as an alternative to hypodermic needle syringes to deliver medicaments through the patient's skin into the underlying tissues. Instead of piercing the skin with a hollow needle, a needleless injector employs a high pressure piston pump to "fire" a fine jet of liquid medicament with sufficient force to puncture the skin, and the remaining medicament is injected through the hole thus formed.
The technique was first used commercially in the 1950's, and is particularly appealing to those who fear needles--so called needlephobia. More recently there has been a surge of interest in needleless injection because it eliminates needlestick injuries, eliminates cross-contamination, and is potentially superior for self administered injections.
One application for needleless injectors is the treatment of erectile dysfunction. Not surprisingly, men suffer a high incidence of needlephobia, since the normal treatment is to inject the medicament with a 12 mm long needle directly into the corpus cavernosum. There have been attempts recently to inject with a needleless injector, but injectors currently available require considerable skill to operate, and the placement requirements for the injector are critical. It is important that the injection misses any superficial blood vessels, and the force of the nozzle on the skin must not be too high, as this would compress the tissues to an undesirable extent and possibly cause the jet of medicament to pass through the corpus cavernosum. Equally, the force of the nozzle on the skin must not be too light or non-existent, since this could result in wasted injectate, bruising or tearing of the skin, haematomas, and poor therapeutic effect. Most currently available needleless injectors are fairly large--typically 160 mm long by 18 mm diameter, and the delivery orifice in the nozzle is difficult to place with the required accuracy, because the surrounding structure obscures the injection site. Most injectors are operated by pressing the nozzle onto the skin and actuating a release button which is usually placed on the opposite end of the injector to the nozzle: at the instant of pressing the injection release button there is invariably a reaction jerk of the injector body which cart shift the position of the nozzle at the instant of injection. Furthermore there is no means provided to ensure that optimum nozzle to skin force is achieved before operating the injector, and variable results are obtained. The present inventor has co-pending applications (e.g. PCT/GB94/01608) in which many of these problems are avoided by arranging for the injector to be triggered in response to a predetermined force of the nozzle on the skin, Nevertheless, with some applications such as the treatment of erectile dysfunction, the injection site is extraordinarily soft and motile, requiring precise positioning, and further skill and care is required.
Another application for needleless injectors is the treatment of keloids--or raised scar tissue. In this case a number of injections are made over the area of the keloid, and it is important to position these carefully. Again, no devices are known to the present inventor which help to improve the positioning accuracy.